4 Wheel Drive/SUVs: 4 wheel drive not working, cv joints, vacuum problem

Question
i have a 96 ford explorer when in 4 wheel drive the front wheels don't pull tho the front axal turns

Answer
Rob,
Are the front axles turning, or is it just the driveline?  
You may have a vacuum problem at the differential.  Most front differentials in this model use a vacuum system to lock in the front end.  When this fails, you lose front wheel drive.  The problem components are:  shift motor, vacuum switch or the vacuum harness/vacuum lines.  This is the most common failure when you lose front wheel drive.
Other possible problems are a broken front axle, then the front end will not put power to the front wheels.  Check out the front axles and axle u-joints/cv joints.
Other problem components could be:  transfer case electric shift motor, shift relay to electronic clutch, transfer Case gear sensor switch, etc.
I would suggest that you purchase a repair manual from a local auto parts store.  These are great for trouble shooting and are not that expensive.

Carl